1. Field of the Invention
The invention relates to a hydraulic valve that is embodied, in particular, as an (proportionally) adjustable pressure control valve. The hydraulic valve comprises a solenoid with a coil and a movable armature, which is located in an armature chamber which is surrounded by the coil and axially delimited by a pole shoe projecting into the coil and which is connected to an armature plunger which penetrates a central bore of the pole shoe. The hydraulic valve also comprises a hydraulic part comprising a valve housing formed as a mounting bushing which rests axially against the pole shoe and has an axially extending valve bore which is connected by at least one radial bore with a low pressure area surrounding the valve housing. The hydraulic part comprises a valve slide positioned in the valve bore and axially movable by the armature via the armature plunger, wherein a fluid chamber is formed between the pole shoe and the valve housing from which an open fluid connection extends, on the one hand, to the armature chamber and, on the other hand, to the low pressure area surrounding the valve housing.
2. Description of the Related Art
A pressure control valve with a solenoid and with a hydraulic part, comprising a valve housing formed as a mounting bushing and having an axially extending valve bore and a valve slide axially moveably positioned in the valve bore, is known from, for example, German patent document DE 41 22 376 C2. The valve bore is connected at three axially spaced apart locations via radial bores with the outer area of the valve housing. Closest to the solenoid is a low pressure area via which the pressure medium can flow out to a tank. Farthest from the solenoid is a high-pressure area to which pressure medium is supplied from a pressure medium source. Between the two aforementioned areas, the control pressure area is located in which the valve controls a pressure whose level depends on the level of the current flowing through the coil of the solenoid. The control pressure acts actually on an active surface of the valve slide in the sense of a connection of the control pressure area with the low pressure area and increases to such a value where a balance of the forces acting on the valve slide and generated by the solenoid, the control pressure, and, as the case may be, a spring optionally present in the arrangement is reached.
The pressure control valve shown in DE 41 22 376 C2 is one with an increasing characteristic line. This means that the control pressure increases with increasing strength of the current flowing through the coil of the solenoid. With such a pressure control valve the force generated by the control pressure and the force generated by the solenoid act in opposite directions on the valve slide. One end face of the valve slide and the hollow chambers of the solenoid are connected by a fluid connection with the low pressure area. This fluid connection is realized by an axial bore which is provided eccentrically to the valve bore in the valve housing and opens into one of the radial bores which extend between the valve bore and the low pressure area provided externally on the valve housing.
It is an object of the present invention to further develop a hydraulic valve of the aforementioned kind such that the pressure forces acting on the valve slide depend, if at all, only minimally on whether and how pressure medium flows through the valve.
In accordance with the present invention, this is achieved in that for a hydraulic valve of the aforementioned kind the fluid connection between the fluid chamber, which is positioned between the pole shoe and the valve housing, and the low pressure area is external to a radial bore which is positioned in a fluid connection between the valve bore and the low pressure area. In the hydraulic valve according to the invention the change of the static pressure in the pressure medium, which results from the change of the pressure medium flow, has only a very minimal effect on the pressure present on the one end face of the valve slide and in the hollow spaces (chambers) of the solenoid. Accordingly, the force level at which a force balance is reached on the valve slide depends little on whether pressure medium flows from the valve bore to the low pressure area external to the valve housing or not. When the hydraulic valve is a pressure control valve, the control pressure is very precisely maintained. In general, the hollow spaces (chambers) in the solenoid and the space or chamber in front of one of the end faces of the valve slide are not directly coupled to the pressure medium flow occurring within the radial bore in which dirt particles may be present so that with the embodiment of a hydraulic valve according to the invention soiling of the aforementioned spaces (chambers) can be prevented and a long service life of the hydraulic valve can thus be ensured.
It is especially advantageous when the pressure medium flows out at a certain location of the circumference from the low pressure area in the form of a peripheral channel and a radial compensation bore, which is positioned in the fluid connection between the fluid chamber and the low pressure area, opens opposite to the outflow location into the low pressure area. Opposite the outflow location the effect of the flowing pressure medium on the static pressure can be maintained at an especially minimal level. Especially preferred is an embodiment with an odd number of uniformly circumferentially distributed radial outflow bores between the low pressure area and the valve bore, wherein one of these radial outflow bores is aligned with the outflow location and the radial compensation bore is positioned opposite to this radial outflow bore. In particular, the radial compensation bore opens into the peripheral channel at a location where no pressure medium flows so that the opening of the radial compensation bore is not subjected to pressure medium flow which permanently conveys dirt particles from the hydraulic circuit which could reach the dead spaces of the flow in the solenoid and a location in front of the end face of the valve slide.
According to another preferred embodiment, the armature plunger in the pole shoe is seal-tightly and slidingly guided in the pole shoe, and an axial bore of the armature plunger is positioned in the fluid connection between the fluid chamber and the armature chamber in the solenoid, wherein the axial bore is open at the end face of the armature plunger positioned within the anchor chamber. In this manner, the channel between the fluid chamber and the armature chamber is very long. The compensation for the increase or decrease of the free volume of the armature chamber by inflow or outflow of pressure medium into or out of the armature chamber as a result of the movement of the armature results in a displacement of the pressure medium column only within the channel between the armature chamber and the fluid chamber so that the armature chamber is effectively protected against soiling. The fluid connection between the two partial armature chambers on both end faces of the armature is realized in an advantageous manner by at least one channel between the armature and the portion of the armature plunger inserted into the armature, wherein preferably the armature plunger has a circular outer periphery and the channel is formed by a deviation of the cross-section of the central throughbore in the armature from a circular shape. The channel preferably extends axially.
In a pressure control valve the force balance acting on the valve slide is not only affected by the low pressure acting on the one end face of the valve slide but also by the control pressure present at the other end face. According to another preferred embodiment, in which the hydraulic valve is a pressure control valve which is proportionally adjustable by a solenoid and which comprises a valve housing embodied as a mounting bushing having an axially extending valve bore which is connected by at least one radial bore, respectively, with a peripheral outer high-pressure area or chamber, a peripheral outer low pressure area or chamber, and a peripheral control pressure area or chamber positioned axially between the high pressure chamber and the low pressure chamber, wherein a valve slide is positioned in the valve bore and is loadable in the sense of a connection of the control pressure chamber with the high-pressure chamber via a solenoid force and in the sense of a connection of the control pressure chamber with the low pressure chamber by the control pressure, the control pressure present at the one end face of the valve slide is affected especially minimally by the pressure medium flow when a fluid path between the control pressure chamber and a pressure chamber positioned in front of the one end face of the valve slide is positioned external to the radial bores connecting the control pressure chamber and the valve bore. With respect to the control pressure chamber and the pressure chamber at the one end face of the valve slide the same principle is applied as with the low pressure chamber and the other end face of the valve slide.
It should be mentioned in this context that the effect of a pressure medium flow onto the force balance on the valve slide can be reduced already by the mere application of the features disclosed in the preceding paragraph so that, in principle, an embodiment without realizing a fluid connection between the fluid chamber and the low pressure chamber external to the radial bore, which provides a fluid connection between the valve bore and the low pressure area, is possible.
The fluid path comprises an axial blind bore which opens at the end face of the valve housing and a radial bore open toward the control pressure chamber and connected with the axial bore and positioned so as to end at a spacing to the valve bore. Preferably, the control pressure area, at least after mounting of the valve housing in a mounting or receiving bore, has a certain outflow and inflow location and the radial bore of the fluid path opens opposite to the outflow location into the control pressure area.
An odd number of radial bores uniformly circumferentially distributed in the valve housing extend between the control pressure area and the valve bore, wherein one of these radial bores is aligned with the outflow location and the radial bore of the fluid path is positioned opposite to this radial bore.